RF (radio frequency) communication systems that act to maximize spectrum efficiency through frequency reuse include cellular radiotelephone systems, some types of trunked communication systems, among others. A common feature that these systems generally share is the division of a service area into smaller areas known as "cells."
Within each cell, a group of relatively low power base stations provides RF communication services to subscribers within that cell over a group of RF channels. Because of the low power, the same group of RF channels may be reused only a short distance away to provide communication services to subscribers in another (although not generally adjacent) cell.
Although offering distinct advantages in terms of spectrum efficiency, a system of the type just described demands considerable investment in infrastructure. Because of the relatively small cell size, a large number of cells may be required to provide adequate service over a large coverage area, and each cell requires a number of base stations, a controller, and an antenna system.
The type of antenna system selected for use within a cell is important both for maximizing system efficiency and for effectively tailoring system operation for particular categories of users. In many systems, each cell is further divided into sectors, multiplying at least the receive antenna requirement for the cell by the number of sectors selected. In a commonly used configuration, each cell is divided into six equal sectors, with each sector having its own directional receive antenna with a radiation pattern closely approximating the sector shape. A single transmit antenna having an omnidirectional radiation pattern is used for transmission into all sectors of the cell.
In other cell configurations, the cell may be divided into sectors for transmitting, as well. This type of system is useful for dealing with cells having irregular boundaries caused, for example, by natural or man-made obstructions. Omnidirectional transmit patterns, in contrast, are most often employed where the desired coverage pattern is approximately circular in shape.
Naturally, antenna systems used in sectored cells are directional antennas. Although the radiation patterns of these antennas are selected to approximate the sector shape, the patterns are not generally easy to alter after installation. A need to alter the radiation pattern may arise based upon studies of system performance, newly constructed obstacles to RF propagation, altering of the shapes of adjacent cells, or for a variety of other reasons.
It may even be required that cell boundaries be altered as a function of time. During periods of relatively low usage, in the evenings and on weekends and holidays, for example, overlapping coverage areas can be created by extending the radiation patterns of the antennas slightly into adjacent cells. This increases the number of channels available to users in the overlap areas, and minimizes the need for hand-offs, but it also increases the likelihood that co-channel interference may occur. During peak periods, when many channels are in use providing service to a relatively large number of users, the radiation patterns should be restored to a state that minimizes adjacent cell overlap.
Of course, extension of radiation patterns can be done with power control, but increasing the power of the RF signals transmitted by the antenna directly impacts the likelihood of undesired interference. Another way of altering antenna radiation patterns is to physically move the antennas themselves, but this is difficult to do after initial installation. It is possible, of course, to provide a mechanism to alter an antenna's azimuth and elevation, much the same way a radar antenna is moved, but such mechanisms are expensive, and the mechanical linkages required to support such movement would degrade the structural integrity of the antenna mounting system.
Accordingly, a need arises for an antenna system that provides an economical and easily manipulated adjustment to its radiation pattern without compromising the integrity of its mechanical mounting structure.